1. Field of the Invention
The present invention relates to a split engine control system which operates a multiple cylinder internal combustion engine using only some of the cylinders under light load conditions, and more particularly to an emission control system for operating the engine in a partial cylinder mode of operation.
2. Discussion of Related Art
It is well known in the art that fuel economy is achieved under light load conditions by operating a multiple cylinder internal combustion engine using only some of the cylinders and allowing these cylinders to operate at their maximum efficiency. When the engine load is relatively heavy, the engine is operated with all of the cylinders. The total number of cylinders is thus split into a first cylinder group which is always activated during the engine operation and a second cylinder group which is deactivated and allowed to pump fresh air therethrough under light load conditions. This method of engine operation is known as a split engine control system.
In order to reduce exhaust gas emissions of the engine employing such system, various emission control systems have been presented by the same inventors of the present invention and others. For example, U.S. Pat. No. 4,256,074, issued May 17, 1981, discloses an emission control system having an exhaust passage which consists of first and second upstream passages connected to the first and second cylinder groups, respectively, and a common downstream passage. A first three-way catalytic converter is disposed in the first exhaust passage with a first exhaust gas sensor, and a second three-way catalytic converter is disposed in the common downstream exhaust passage with a second exhaust gas sensor. When the engine is operated in a full cylinder mode, the engine is feedback controlled by a signal from the second exhaust gas sensor so as to maintain the ratio of the air-fuel mixture at a stoichiometric point, and the exhaust gases are purified by both three-way catalytic converters. When the engine is operated in a partial cylinder mode, the engine is feedback controlled by a signal from the first exhaust gas sensor, and the exhaust gases from the activated first cylinder group are purified by the first three-way catalytic converter.
But this arrangement has a transient problem when the engine is switched from partial to full cylinder mode, because the temperature of the second catalytic converter may be lowered below a minimum active temperature by fresh air pumped through the deactivated second cylinder group during the partial cylinder mode and the second catalytic converter may become inactive. On the other hand, U.S. Pat. No. 4,245,471, issued Jan. 20, 1981, discloses an emission control system having the same exhaust passage system as described above except using only one exhaust gas sensor which is disposed in the common downstream exhaust passage upstream of the second three-way catalytic converter. When the engine is operated in a full cylinder mode, the engine is feedback controlled by a signal from the exhaust gas sensor and the exhaust gases are purified by both first and second three-way catalytic converters. When the engine is operated in a partial cylinder mode, the air-fuel mixture supplied to the activated first cylinder group is enriched with respect to the stoichiometric point. This allows the first three-way catalytic converter to promote the reduction of the NOx component of the exhaust gases, and allows the second three-way catalytic converter to promote the oxidation of the remaining HC and CO components of the exhaust gases mixed with the fresh air. The heat of reaction in the second catalytic converter keeps its temperature high and keeps it active during the partial cylinder mode.
But this arrangement has a disadvantage on fuel economy during the partial cylinder mode as compared with the first arrangement, because the rich burn operation requires enriched air-fuel mixture with respect to the stoichiometric point.